1. Field of the Invention
The present invention relates to a positive resist composition suited for use as a second resist composition in a double patterning process in which a first patterning is performed using a first resist composition and a second patterning is performed using a second resist composition, and a method of forming a resist pattern.
Priority is claimed on Japanese Patent Application No. 2008-006293, filed Jan. 15, 2008, and Japanese Patent Application No. 2008-097432, filed Apr. 3, 2008, the contents of which are incorporated herein by reference.
2. Description of Related Art
A technique of forming a fine pattern on a substrate and processing a lower layer of the fine pattern by performing etching using it as a mask (pattern forming technique) is widely used in the production of ICs of the semiconductor industry and attracts special interest.
The fine pattern is usually composed of an organic material and is formed by techniques, for example, lithography and nanoinprinting techniques. In lithography techniques, for example, a resist film composed of a resist composition containing a base material component such as a resin is formed on a support such as a substrate, and the resist film is subjected to selective exposure of radial rays such as light or an electron beam through a mask having a predetermined pattern (mask pattern), followed by development thereby forming a resist pattern having a predetermined shape on the resist film. Then, a semiconductor element is produced by processing the substrate through etching using the resist pattern as the mask.
A resist composition in which the exposed portions become soluble in a developing solution is called a positive-type, and a resist material in which the exposed portions become insoluble in a developing solution is called a negative-type.
In recent years, advances in lithography techniques have lead to rapid progress in the field of pattern miniaturization. Typically, these miniaturization techniques involve shortening the wavelength of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are now starting to be introduced in mass production. For example, it becomes possible to form patterns at a resolution of 45 nm by lithography techniques using ArF excimer lasers. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a wavelength shorter than these excimer lasers, such as F2 excimer lasers, electron beams, extreme ultraviolet radiation (EUV), and X rays so as to further improve resolution.
Resist compositions for use with these types of exposure light sources require lithography properties such as a high level of sensitivity to these types of exposure light sources and a high resolution capable of reproducing patterns of minute dimensions. As a resist composition which satisfies these conditions, a chemically amplified resist composition is used, which includes a base material component that exhibits a changed solubility in an alkali developing solution under action of an acid and an acid generator that generates an acid upon exposure (see, for example, Patent Document 1). For example, a chemically amplified positive resist contains, as a base material component, a resin which exhibits increased solubility in an alkali developing solution under action of acid. In the formation of a resist pattern, when an acid is generated from the acid generator upon exposure, the exposed portions become soluble in an alkali developing solution.
Polyhydroxystyrene (PHS) or derivative resins thereof in which the hydroxyl groups are protected with acid-dissociable, dissolution-inhibiting groups (PHS-based resins), which exhibit high transparency to a KrF excimer laser (248 nm) are mainly used as the base resin component of chemically amplified resists. Resins that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are now widely used as base resins for resists (ArFs that use ArF excimer laser lithography, as they exhibit excellent transparency in the vicinity of 193 nm.
As one of techniques for further improvement of resolution, for example, there is known a lithography technique, so-called liquid immersion lithography (hereinafter may be referred to as liquid immersion exposure) in which exposure (immersion exposure) is performed in the presence of a liquid having a higher refractive index than that of air (liquid immersion medium) interposed between an objective lens of a stepper and a sample (see, for example, Non-Patent Document 1).
It is said that the liquid immersion exposure can attain high resolution similar to that attained by using a light source having a shorter wavelength or a high NA lens even when a light source having the same exposure wavelength is used, and also does not cause a decrease in the depth of focus. The liquid immersion exposure can be performed by application of an existing stepper. Therefore, it is expected that the liquid immersion exposure can realize formation of a resist pattern having high resolution and excellent depth of focus at low cost. Also, the liquid immersion exposure has attracted special interest recently as those capable of exerting a great effect on the semiconductor industry in view of cost and lithography characteristics such as resolution in the production of semiconductor elements that requires large equipment investments.
It is considered that the liquid immersion exposure is effective for formation of any pattern shape and also can be used in combination with super-resolution techniques such as phase-shift and modified illumination techniques that are now studied. As the liquid immersion exposure technique, a technique using an ArF excimer laser as a light source is now mainly studied. As the liquid immersion medium, water is mainly studied.
As one of lithography techniques, there is newly proposed a double patterning process in which a resist pattern is formed by performing patterning two or more times (see, for example, Non-Patent Documents 2 and 3).
It is considered that this double patterning process enables formation of a resist pattern having higher resolution than that of a resist pattern formed by single patterning by forming a first resist pattern on a substrate using a first resist composition, and forming a pattern on the substrate, on which the first resist pattern is formed, using a second resist composition.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2003-241385
Non-Patent Document 1: Proceedings of SPIE, Vol. 5754, pp.119-128 (2005)
Non-Patent Document 2: Proceedings of SPIE, Vol. 5256, pp. 985-994 (2003)
Non-Patent Document 3: Proceedings of SPIE, Vol. 6153, pp. 615301-1-19 (2006)
In the double patterning process described above, when the second resist composition is applied, the first resist pattern is likely to be adversely influenced. Therefore, for example, there arise problems that the first resist pattern is partially or entirely dissolved in the solvent of the second resist composition to cause thickness, leading to shape deterioration, and that the first resist pattern and the second resist composition are dissolved in each other to cause so-called mixing, leading to unsatisfactory formation of a resist pattern.
It is considered that these problems can be avoided by using, as the second resist composition, a resist composition dissolved in a solvent in which the first resist pattern is hardly soluble. Therefore, when a positive resist composition is used as the first resist composition, a negative resist composition having high solubility in an organic solvent with low compatibility with the positive resist composition has conventionally been used as the second resist composition. As the organic solvent with low compatibility with the positive resist composition, for example, alcohol-based solvents and ether-based organic solvent having no hydroxyl group have been used.
When the positive resist composition is used as the second resist composition similar to the first resist composition, it is necessary to preliminarily protect the first resist pattern using a freezing agent and thus there is a problem such as poor workability since the number of steps is larger than that in the case of using the negative resist composition. An acrylic resin to be used as a base resin of an ArF positive resist is now often used in the form of a positive resist composition prepared by dissolving it in organic solvents such as propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monomethyl ether (PGME), cyclohexanone, 2-heptanone and lactic acid ethyl lactate (EL) for the following reason. That is, when the positive resist composition is directly applied on the first resist pattern, it dissolves the first resist pattern.
The present invention takes the above circumstances into consideration, with an object of providing a positive resist composition which can be used as a second resist composition in the formation of a resist pattern through double patterning using a positive resist composition as a first resist composition, and a method of forming a resist pattern.